The badge on the left pocket is the Infanterie-Sturmabzeichen (Infantry Assault Badge), awarded for participating in three infantry assaults or reconnaissance patrols. The medal appears to be a Kriegsverdienstkreuz mit Schwertern, or War Merit Cross with Swords, for service in combat (hence the swords) above and beyond the call of duty, but not enough to merit the Iron Cross. I don't see any rank, so he was probably a private when this picture was taken.

Your great uncle wasn't Croatian, was he?

Hello there! As your question is related to looking for identification/information regarding military personnel, our Guide on Military Identification may be of use to you. It provides a number of different resources, including how to request service records from a number of national agencies around the world, as well as graphical aids to assist in deciphering rank, unit, and other forms of badges or insignia. While the users here may still be able to lend you more assistance, hopefully this will provide a good place to start!

That is an officers uniform but I can't see his eppulet clearly.... Probably a lieutenant 2nd class (leutnant).

Has iron Cross 2nd class.

Can't make out the other details.